Certain driving conditions can produce elevated engine speed and load conditions that may increase engine exhaust feed-gas temperature. If sufficient temperatures reach emission-reducing catalysts for prolonged periods of time, reduction in long-term catalyst efficiency may result. Elevated catalyst temperatures can produce wash coat sintering and substrate deterioration, reducing wash coat surface area and catalyst efficiency. Catalyst temperature is commonly controlled using fuel enrichment because fuel enrichment reduces feed gas temperature without affecting catalyst long-term efficiency.
One method to control catalyst temperature by adjusting fuel is described in U.S. Pat. No. 4,960,083. This method provides generating a changeover-instructing signal for changing valve timing to the low speed valve timing in both the lower and higher engine speed regions, when an abnormality is detected and enriching the air-fuel ratio of an air-fuel mixture to be supplied to the engine. The enriching of the air-fuel ratio is carried out by multiplying a basic fuel injection period by a correction coefficient.
The inventor herein has recognized several disadvantages of this approach. Namely, the approach changes fuel delivery and valve timing without regard to engine torque. When fuel is enriched and valve timing changed, engine torque can change. Enrichment from stoichiometry increases engine torque until the Lean Best Torque (LBT) limit is reached; from that point, additional fuel no longer increases torque. Advancing or retarding valve timing may also increase or decrease torque depending on present valve timing. Since engine torque is not considered by the before-mentioned approach, the approach may introduce drivability concerns resulting from any torque variation.
Furthermore, the inventor has recognized that the approach increases engine fuel consumption while the catalyst temperature is being controlled by air-fuel enrichment. While controlling catalyst temperature is important to protect a catalyst from deterioration, fuel consumption affects emissions and operating costs.